Colicin K causes at least three proteins to disappear from the membranes of Escherichia coli. The fate of each protein, hydrolysis or solubilization, will be examined, and the mechanism by which the colicin alters the proteins or the membrane residue will be sought. The altered protein composition seen on SDS polyacrylanide gel electrophoresis will serve as an assay for in vitro activity of the colicin. The proteins removed by colicin will be purified from normal cells. Their ability to restore the normal premeability barrier and energy-dependent functions cuh as NADH-NADP transhydrogenase and active transport to membranes of colicin-treated cells will be tested. The processes necessary for survival of colicin-treated cells will be Studied. Desorption or degradation of I125-colicin may be required, particularly in cells having an active Mg2 ion - Ca2 ion ATPase. The colicin may activate an ionophore of broad specificity. Extracts of colicin-treated cells will be tested for the ability to swell membrane vesicles in buffers containing appropriate cations. If an ionophore exists, larger amounts will be prepared, purified and characterised.